1. Field of the Invention
The manufacture of blocked or masked polyisocyanates is known and is described in Houben-Weyl "Methoden der organischen Chemie" ("Methods of Organic Chemistry"), Volume 14/2, page 61-70.
Reaction products of hexamethylene-1,6-diisocyanate with tert.-butanol, phenol, acetoacetic acid ethyl ester, malonic acid esters, acetylacetone, phthalimide, imidazole, hydrogen chloride, hydrogen cyanide and caprolactam are known.
The above aliphatic isocyanate was reacted with compounds which at elevated temperature split off again, with liberation of the isocyanate group. Such products are described as isocyanate-eliminating compounds or "moderators". In contrast to the free diisocyanates, it is possible to mix such masked diisocyanates with substances or solvents containing hydroxyl groups, without a reaction taking place. It is thus possible, using masked polyisocyanates, to prepare storageable mixtures with products containing hydroxyl groups, such as higher-molecular polyesters or polyethers, which only give the desired isocyanate reactions at an elevated temperature. The masked polyisocyanates are of great importance both for the manufacture of rubbery-elastic products via intermediates which can be stored, and for the manufacture of wire lacquers, as well as in the textile field. The effect of eliminating the isocyanate results from the fact that practically all adducts formed from isocyanates at a moderately elevated temperature again decompose at higher temperatures, equilibria being set up. The setting up of these equilibria is accelerated by addition of tertiary bases.
2. Prior Art
Many of these masked polyisocyanates show unsatisfactory crosslinking at low stoving temperatures when used in combination with polymers containing hydroxyl groups. However, masked isocyanates which decompose again at lower temperatures, given decomposition products which, because of their toxicity, must not be used for stoving lacquers.
A known adduct, manufactured from hexamethylenediisocyanate and acetoacetic acid ethyl ester admittedly does not give any decomposition products which are particularly toxic, and also decomposes again at a low stoving temperature. The known adduct has a melting point of 81.degree. - 82.degree. C and therefore must be prepared in the melt above the melting point, say at about 90.degree. C, which results in an undesirable yellow coloration, which can only be removed by recrystallisation. This solid adduct tends to crystallise and results in an undesirable inhomogeneity of the lacquer solutions prepared therewith. Thus, for example, when combining polymers, containing hydroxyl groups, with the known adduct, the stoved lacquers have greatly worsened film properties.
It is the object of the invention to provide completely or partially blocked diisocyanates which at room temperature are in the form of a liquid and are soluble in inert solvents customary in the lacquer industry.
It is a further object of the present invention to provide blocked liquid polyisocyanates which in a temperature range of 80.degree. - 130.degree. C, preferably 90.degree. - 130.degree. C, in the presence of polymers containing hydroxyl groups, give crosslinked reaction products.